1) Field of the Invention
The present invention relates to a mobile terminal, a program for controlling a mobile terminal, a program for a call-management server, and a call-management process for performing wireless communication, and in particular to a mobile terminal, a program for controlling a mobile terminal, a program for a call-management server, and a call-management process which can use a plurality of communication means.
2) Description of the Related Art
In recent years, use of the VoIP (Voice over Internet Protocol) technology for making a voice call through the Internet or an intranet is widely spreading. The VoIP technology enables transmission of voice information and voice calls between terminals by use of data communication protocols in computer networks. Therefore, the VoIP technology can be applied to any networks in which predetermined data communication protocols are used, regardless of whether the networks are wired or wireless.
Therefore, it is possible to realize wireless IP (Internet Protocol) telephone equipment by implementing in telephone equipment the function of performing data communication by use of a wireless LAN (Local Area Network). The use of the wireless IP telephone enables to make a call in the vicinity of a base station of the wireless LAN. In addition, when a roaming function is implemented, the use of the wireless IP telephone enables to make a call while the caller moves between base stations, for example, as disclosed in Japanese Unexamined Patent Publication No. 2002-125069.
Further, the above functions of the wireless IP telephone can be implemented in a mobile telephone used in a mobile telephone network or a PHS (personal handyphone system) network. Furthermore, it is possible to implement the functions of the mobile telephone or a PHS terminal in a personal digital (data) assistant (PDA) having the functions of the wireless IP telephone.
Moreover, the VoIP technique can be used on a telephone line in a mobile telephone network, a PHS network, or the like. However, the narrow bandwidth of the telephone line is likely to cause a voice interruption or a great delay. Therefore, it is necessary to properly use the VoIP technique in the following manner. That is, the VoIP technique is used in a network having a great bandwidth such as a wireless LAN, and is not used on a telephone line in a mobile telephone network, a PHS network, or the like which has a narrow bandwidth. Normally, one of the VoIP call function and the call function using a telephone line is selected based on the condition of communication or the like before a call is initiated.
In the case of the mobile communication, the condition of communication varies with movement of the mobile terminal. Therefore, the condition of communication can deteriorate during a call.
However, according to the conventional techniques, it is impossible to dynamically switch between the function of the call through a telephone line and the function of the VoIP call through a computer network according to the circumstances so as to smoothly continue a call.
In addition, when both of the function of realizing the call through a telephone line and the function of realizing the VoIP call through a computer network are on standby in a mobile terminal, the power consumption becomes extremely great. For example, the wireless LAN devices consume more electric power than the mobile telephones. The causes of the great power consumption of the wireless LAN devices are as follows.
The MAC-layer processing in the wireless LAN devices is performed in accordance with CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), which is different from TDMA (Time Division Multiple Access) in that each wireless LAN device must detect (listen to) signals from other terminals during idling. Therefore, the power consumption in a typical wireless LAN device during idling is as great as 1,150 mW in the case where the transmission power is 1,650 mW and the reception power is 1,400 mW. On the other hand, the power consumption in a typical card-type PHS device during idling is tens of milliwatts. Although a function called PSM (Power Saving Mode), which is a function of suppressing the power consumption of a device by making the device sleep as long as possible during idling, is standardized for the wireless LAN devices, many wireless LAN devices are not implemented with the PSM function.
As explained above, when the VoIP function of a mobile telephone which is network-connected to a wireless LAN is on standby, the battery in the mobile telephone runs out in a short time. Therefore, it is desired that only one of the function of realizing a call through a telephone line and the function of realizing a VoIP call through a computer network stands by.